Individuals are considered “colorblind” or “color deficient” when they are unable to distinguish colors that are distinguishable by a color-normal observer. Color-normal observers require three adjustable primaries to match any given light stimulus and use specific ratios in their matches. Approximately 8% of male observers have some form of colorblindness. Approximately 6% of male observers are considered “anomalous trichromats”, still requiring 3 primaries for color matches, but having lower color discrimination performance than color-normal observers. Approximately 2% of male observers exhibit “dichromatic” colorblindness, meaning that they use only two primaries to perform color matches.
When colorblind observers view soft- and hard-copy material designed for color-normal observers, certain color differences, visible to the color-normals, will not be visible to the color deficient observers. This can result in difficulties reading colored text, locating colored objects, or discriminating business graphics or map content distinguished primarily by color. A concrete example of this problem is the typical usage of red and green differentiation of graphical content, which for the more common types of colorblindness will appear to be the same hue.
The proposals to address this problem have not been totally effective. One method is to supply the colorblind observer with optical devices that differentially apply color filters to the two eyes. The goal is to restore discriminability to colors that the colorblind observer is unable to distinguish. While such a method may improve discriminability for some colors, it will also shift the discriminability problem to other colors. Another method available to colorblind computer users is to adjust or select a color lookup table (“LUT”) to remap colors so that problematic colors are mapped to ones with higher color contrast. This method can be applied globally, affecting the entire display, by palletizing the entire display content. If the entire display is not palletized, the manipulation will only be effective on palettized content to which the LUT is applied. The palette can introduce other artifacts such as contouring due to the limited palette size.
The problem of colorblindness has also been addressed by providing methods for simulating the appearance of the different types of color deficiencies. Such a method can be used by a graphic designer to adjust the display content so as to maintain color discriminability for colorblind observers. Several similar methods of simulating dichromatic color appearance have been proposed. In each, a locus is selected for the xy chromaticities that represent the same color sensations for the dichromat and trichromat. Chromaticities corresponding to pixels in an image are then projected onto the locus to generate an image that will be seen as equivalent by the given dichromat. The chromaticities in the resultant image will be confined to the locus and will approximately equate the color sensations for dichromatic and trichromatic viewers. The final implementation of the simulation may be on the pixel color values or on a set of color palette entries.
The cause of colorblindness is related to processing in the three classes of sensors in the eye. These are referred to as L, M or S cones depending on whether they are most sensitive to long, middle or short wavelengths of light, respectively. The most typical forms of colorblindness are related to signaling by the long- or medium-wavelength sensitive cone classes. In anomalous trichromats, the cone spectral sensitivities are different from the color-normal observer, while in a true dichromat, a cone class is missing. A rarer form of colorblindness is related to signaling by the short-wavelength sensitive cone class.
FIG. 1 illustrates the sensitivities of the three L (2), M (4), and S (6) cone classes found in a trichromatic, color-normal eye. In dichromats, one of these sensor classes is either replaced with one of the remaining classes or a signal transduction problem prevents normal processing of signals from that cone class. The result is that color vision becomes two-rather than three-dimensional. While a trichromat requires three distinct lights to match an arbitrary light, dichromats require only two lights and an intensive adjustment. In FIG. 1, normalized short-wavelength is shown at (S) 6, middle-wavelength at (M) 4, and long-wavelength at (L) 2 for Smith-Pokorny cone sensitivities.
Dichromats are referred to as “protanopes”, “deuteranopes”, or “tritanopes”, depending on whether the L, M, or S cone class is deficient, respectively. Experiments have been conducted that identify the confusable colors for each type of dichromat. See Wyszecki, G.; W. S. Stiles (1982) Color science: Concepts and methods, Quantitative data and formulae, Wiley; hereby incorporated herein by reference.
In the colormetry system CIEXYZ advanced by the Commission International de l'Eclairage (CIE), an independent colorimetry system called CIEXYZ is presented. In this tristimulus system, X, Y, and Z denote the mixture of light in a three primary system that characterizes colors that match to the color-normal observer. In order to facilitate the interpretation of data in the CIEXYZ system, CIE developed the ClExyY system, where the two variables x and y describe chromaticity while the variable Y describes the intensive attribute and is referred to as the luminance of the color.
FIG. 2 illustrates the confusion lines for protan, deutan, and tritan dichromats on a CIE xy chromaticity diagram. Points along each line represent stimuli that produce the same response from the remaining cone classes. To a trichromat, these points are distinguishable; to the given dichromat, they are not.
Anomalous trichromats possess three photopigments, but one or more of the pigments are shifted relative to the color-normal observer. The terms “protanomalous”, “deuteranomalous”, and “tritanomalous” are used, according to the affected photopigment. For these anomalous trichromat observers, color discrimination may be nearly as good or worse than a color-normal trichromat, depending on the degree of separation between shifted photopigments. As such, color enhancement methods designed for dichromats may also alleviate poor color discriminability for anomalous trichromats.